Cutting tools for forming grooves and performing back turning have been widely used. Such cutting tools may include a breaker groove on a rake face of a cutting insert used in the cutting tool. This breaker groove is for smoothly discharging chips produced when cutting a work material from a cutting portion. The breaker groove is basically disposed so that chips extending from a cutting edge cross the breaker groove. A descending surface on the side of the cutting edge of the breaker groove draws the chips into the breaker groove. An ascending surface on the side opposite the side of the cutting edge of the breaker groove is a wall section into which the chips, having been drawn into the breaker groove, collide, causing the wall section to deform the chips. The deformed chips are quickly discharged from the vicinity of the cutting edge of the cutting tool.
For example, Patent Document 1 discloses a cutting insert (throw-away tip) 40 for back turning that includes a breaker groove 41 on a rake face 42 such as illustrated in FIG. 13. The breaker groove 41 of Patent Document 1 includes an insular portion 43 having a flat height and disposed in a position adjacent to the breaker groove 41 of the rake face 42. The insular portion 43 increasingly narrows on the side of the front cutting edge 44 and has an acute angle shape on the side of the front cutting edge 44.
When back turning is performed using the cutting insert 40 of Patent Document 1, first the cutting edge (the front cutting edge 44 and a lateral cutting edge 45) is advanced in a direction perpendicular to a machined surface of the work material by a distance equivalent to the cut amount, and then machining is performed while advancing the cutting edge rearward. As a result, during back turning, many chips are produced from the side of the front cutting edge 44 when cutting is started during the initial stage, and many chips are produced from the side of the lateral cutting edge 45 when the cutting edge is subsequently advanced rearward. Furthermore, before back turning, groove-forming while advancing the cutting edge frontward may also be performed and, in this case as well, many chips are produced from the side of the front cutting edge 44.
When such a cutting insert 40 that includes the insular portion 43 having a frontward acute angle shape as that in Patent Document 1 is used, the advancing direction of the chips produced from the side of the front cutting edge 44 during the initial stage of back turning and during groove-forming is nearly parallel to the breaker groove 41. In Patent Document 1, the height of the insular portion 43 is greater than a height of the front cutting edge 44 and the lateral cutting edge 45 so that the chips produced during back turning do not pass over the breaker groove 41.